Pupil
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The human eye
The pupil is the central transparent area (showing as black). The greenish-brown area surrounding it is the iris. The white outer area is the sclera, the central transparent part of which is the cornea.'' |
In the
eye, the
pupil is the opening in the middle of the
iris. It appears black because most of the
light entering is absorbed by the
tissues inside the eye.
In
humans and many
animals (but few
fish), the size of the pupil is controlled by involuntary contraction and dilation of the iris, in order to regulate the intensity of
light entering the eye. This is known as the
pupillary reflex. In bright light, the human pupil has a diameter of about 1.5
millimeter, in dim light the diameter is enlarged to about 8 millimeter.
The shape of the pupil varies between
species. Common shapes are
circular or slit-shaped, although more convoluted shapes can be found in
aquatic species. The reasons for the variation in shapes are complex; the shape is closely related to the optical characteristics of the lens, the shape and sensitivity of the
retina, and the visual requirements of the species.
Slit-shaped pupils are found in species which are active in a wide range of light levels. In strong
light, the pupil constricts and is small, but still allows light to be cast over a large part of the retina.
The orientation of the slit may be related to the direction of motions the eye is required to notice most sensitively (so a vertical pupil would increase the sensitivity of the eyes of a small cat to the horizontal scurrying of mice).
Many
snakes, such as ,
pythons and
vipers, have slit-shaped pupils.
When an eye is
photographed with a
flash, the iris cannot close the pupil fast enough and the blood-rich
retina is illuminated, resulting in the
red-eye effect.
Dilation occurs when signals from
sympathetic fibers (these fibres travel from the sympathetic trunk up to the face) cause the
iris dilator muscles to contract. Damage to the sympathetic tract to the face results in
Horner's syndrome, a symptom of which is excessive pupillary constriction (
miosis) in one eye. This disparity in pupil size is termed
anisocoria.
The human pupil dilates if the person sees something (or someone) of interest or is
aroused. Studies have shown that
humans (especially
females) are judged as more
attractive if their pupils are wide open and more dilated than normal. The name
Belladonna (
beautiful lady) comes from the fabled use of the juices of the
Nightshade plant by
Italian women who would use eyedrops made from an extract of the plant in their eyes in order to enlarge their pupils and make their eyes appear more
beautiful. In some cases gradual
blindness has occurred through the overuse and abuse of belladonna to enhance the beauty of the eyes.
Atropine, a chemical found in belladonna, is used by modern
eye doctors to dilate the pupils so that they can examine the
retina. Excessive dilation is called
mydriasis.
Hallucinogenic drugs, such as
LSD and
psilocybin, and some stimulants such as
cocaine also cause dilation of the pupils.
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The pupils of sheep and goats are horizontal slits. |
When bright light is shone on the eye, it will automatically constrict. This is the
pupillary reflex, which protects the
retina from bright light, and is an important test of
brainstem function. Also, the pupil will constrict if a person sees an object or a person which arouses anger or loathing.
The
oculomotor nerve, specifically the
parasympathetic part coming from the
Edinger-Westphal nucleus, terminates on the circular
iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.
Certain drugs cause constriction of the pupils, such as
heroin.
Another term for the constriction of the pupil is
miosis. Substances that cause miosis are described as
miotic.
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Eye contact*
Synechia
